1. Field of the Invention
The invention relates to the manufacture of tubular fibrous products or shells, intended particularly for the heat insulation of conduits and made of mineral fibers, for example of glass, agglomerated by a polymerized binder. The invention relates more particularly to techniques for winding of felts of mineral fibers around a mandrel of a predetermined length to produce cylindrical shells.
2. Discussion of the Art
According to these techniques, a felt of mineral fibers impregnated with a binder consisting of a polymerizable resin, for example of the melanin-formaldehyde, phenyl-formaldehyde or phenol-urea type, is cut into sections of a predetermined length. Each section of felt is wound around a revolving mandrel while the polymerization of the binder begins, which is then completed in a heated chamber.
This process is particularly known from U.S. Pat. No. 4,153,498 to which special reference is made. In this process the rvolving mandrel, around which the felt of mineral fibers is wound, is heated. This heating of the mandrel facilitates the anchoring of the first layer or wrap of felt. The temperature is selected so that an inner surface of the shell hardened by the polymerization of the binder in the vicinity of the mandrel is formed during the winding time. Thus, as soon as the winding ends, the shell can be separated from its mandrel and can be transferred to a device which assures the smoothing and hardening of the outside surface of the shell. At this stage, it exhibits hardened inner and outer surfaces while apart from these inner and outer surfaces, the polymerization of the binder remains incomplete. Polymerization is then completed homogeneously through the entire thickness of the shell in a heated chamber.
Also according to U.S. Pat. No. 4,153,498, the winding of the felt around the revolving mandrel is performed while maintaining the speed of the mandrel, leading to an accelerated tangential speed. Because of this, the thickness of the shell being formed increases with a constant speed.
This process is perfectly suited for the production of shells of small inside and outside diameters, for which only small lengths of felt of mineral fibers need be wound, for example less than 6 meters. Feeding the winding device should be performed at an accelerated speed, however it is impossible to increase the speed for feeding felt of mineral fibers too much without risking a tearing of the felt which is made more fragile by the fact that the mineral fibers are not yet bound to one another. The maximum feeding speed reached at the end of winding is a function of the outside diameter of the shell and of the rotation speed of the mandrel, and should be less than the speed beyond which the felt might be torn. This calls for a maximum rotation speed of the mandrel, to be inversely proportional to the outside diameter of the shaped shell at the end of winding. This limitation becomes particularly constraining for shells of large outside diameter. Thus, by way of example, if a feeding speed limited to 50 meters per minute is assumed, for a shell with an outside diameter of 400 mm, the mandrel should have a constant rotation speed less than 40 revolutions per minute. With layers of an average thickness of about 0.3 mm, a winding time for a shell of 100 mm total thickness is greater than 8 minutes. The rate of production according to this example would therefore be very low.
According to another important characteristic of U.S. Pat. No. 4,153,498, during the entire time of winding, pressing elements remain in contact with the shell being formed. These pressing elements consist, for example, of three counterrollers placed around the heated revolving mandrel. Simultaneously withdrawing from the axis of the mandrel as the shell is formed, these counterrollers assure, on the one hand, the uniformity of the winding and, on the other hand, the cohesion of the shell. Actually, these counterrollers define uniform lines of contact with the shell being formed, which define the general shape of the shell during the winding time. In addition, by the way their pressure is exerted, the counterrollers avoid any nonuniformity of the layers of wound felt.
In practice, three counterrollers are satisfactory for "small" shells, i.e., shells whose inside diameter is between 12 and 100 mm and whose outside diameter is less than 200 mm. When these limiting values are exceeded, for example for shells whose outside diameter reaches 500 mm, three contact points prove insufficient to define the shape of the shell correctly and the counterrollers no longer assure the desired cohesion. Since the squeezing of the shell is maintained by the counterrollers, the pressure exerted is all the greater if a large portion of the outside surface of the shell is in contact with the counterrollers; in other words, if the surface of each counterroller in contact with the shell is increased. However, this contact surface is limited by the fact that the diameter of the counterrollers cannot exceed such a value that the counterrollers are both tangential to one another and to the heated revolving mandrel which determines the value of the inside diameter of the shell. Of course, it would be possible to increase the number of counterrollers, but their diameter would then have to be reduced for the same reasons of bulk. Because of this, an installation well-suited to the production of shells of small inside diameter would provide shells of average inside diameter and/or of average thickness of poor quality while, reciprocally an installation well suited to the production of shells of average inside diameter would not be able to produce shells of small inside diameter, because no pressure would then be exerted on the first wound layers.
The use of this process of the art for the production of shells of average thickness also runs into an additional difficulty connected with the compressibility of the product. Actually, according to this process, the counterrollers are gradually withdrawn from the axis of the revolving mandrel so that during the entire winding phase, a constant force is exerted on the felt of mineral fibers by the counterrollers. Consequently, the first layers or wraps wound, whose outer surface remains not far from the completely rigid surface of the revolving mandrel, are more compressed than the last wraps which are separated from the rigid mandrel by considerable thickness of compressible felt. Because of the partial elasticity of the felt of mineral fibers, and because of this difference in compression, the pickup of thickness of the shell is greater at the end of winding; the result is a shaped shell whose outside diameter is imperfectly controlled and greater than the expected theoretical diameter.